Vector control technologies for permanent-magnet synchronous electric motors (hereinafter abbreviated as “electric motor”) using an inverter are widely employed in industry. By separately controlling the magnitude and the phase of inverter output voltage, a current vector in an electric motor is optimally controlled, so that torque of the electric motor is fast and instantaneously controlled. Permanent-magnet synchronous electric motors are known as high-efficiency electric motors in comparison with induction motors because no energizing current is needed due to establishment of magnetic field by the permanent magnet and no secondary copper loss is generated due to no rotor current. For that reason, application of permanent-magnet synchronous electric motors to electric railcars has been investigated in recent years.
Subjects with controllers in applying permanent-magnet synchronous electric motors to electric railcars are to realize a stable field-weakening operation up to a high speed range and to achieve a stable transition to a one-pulse mode in which inverter loss can be minimized and voltage applied to the electric motors can be maximized. The one-pulse mode is an operation mode for inverters, in which an inverter outputs, as its output line voltage, square waves having a positive and a negative maximum rectangular voltages of 120 degree durations each that are repeated one after another with a zero voltage period of 60 degrees therebetween, in one cycle, i.e., 360 degrees.
The following method is disclosed in Patent Document 1 as a related prior art. A voltage setting unit is provided that receives a voltage fixing command and a voltage command calculated based on a current command. When the voltage fixing command is input, the voltage setting unit outputs a voltage command as a new voltage command by setting its magnitude to a predetermined voltage set value. A magnetic-field-direction (d-axis) current command is then corrected using a magnetic-field-direction (d-axis) current correcting value obtained by taking a proportional-integral control of the difference between the voltage command calculated from the current command and the new voltage command. A modulation index for the inverter is then calculated from the voltage command to control the inverter, so that a field-weakening operation is performed.
Patent Document 1: Japan Patent Application Laid-Open No. H09-84399 (see paras. [0023]-[0029]).